Fix build without mbedtls [v2]

[aeolio]

What does this PR aim to accomplish?:

Commit 3b45213 (Implement automatic TLS/SSL certificate renewals) introduced functionality that requires mbedtls, but mbedtls is an optional dependency. This leads to the following compilation error:

pihole-ftl-6.5/src/webserver/webserver.c:36:10: fatal error: mbedtls/ssl_ciphersuites.h: No such file or directory
   36 | #include <mbedtls/ssl_ciphersuites.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

How does this PR accomplish the above?:

Use the HAVE_MBEDTLS preprocessor symbol to assert that certificate related code is only included if mbedtls is present in the build.

Link documentation PRs if any are needed to support this PR:

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